Mercruiser 496 (8.1L) overheating

Ok, just got back from a run into the gulf. Here's the data. Water pressure was 3.3 at idle, 10 at 1200, but 33 at 4200 when she dropped into guardian mode. The temp on vesselview was 167 on the stb engine. We measured the temps at the thermostat at 160, temp on the manifolds were 205. The vesselview alarm history said. Overtemp-Engine is overheating, check for plugged inlet. Water pump may be faulty.

From the data above the only number that appears out of line is the high end water pressure. In my understanding that should be down around the 20 range at the high end. My water pump should be good as it's new a year ago and I'm getting pressure. It appears that I have blockage somewhere after the water pump. Our plan is to drain, replace the coolant and clean the heat exchanger and thermostat. We'll run her again and if we don't see an improvement then we'll look into the risers.

We still don't know what sensor tripped the guardian mode. I'm suspecting the water pressure sensor. Does anyone know at what pressure it should trip the guardian mode? Also, is there a way to get more information such as that from vesselview?
 
I would bet its the exhaust manifold sensor on the Port side. I think they alarm @ 195 . Your other temps seem ok and yes your 4200 water PSI is a bit high, but not terrible. Something to consider is what is the sea water temp? If it is cool then when it does get warmer, you will run hotter. I think your plan is spot on. Best of luck.
 
High water pressure indicates pressure side of pump issue, not suction side. I would suspect PS cooler, oil cooler or heat exchanger is plugged.
 
Just to conclude this thread. It ended up being the risers. I remember now when I was looking for a boat that any of the 496's that were 8-10 years old all had their risers replaced.
 
2808670-R1-015-6.jpg
cbruck said:
Just to conclude this thread. It ended up being the risers. I remember now when I was looking for a boat that any of the 496's that were 8-10 years old all had their risers replaced.
Click to expand...
Good that you caught it in time.
 
Well I don't think I'm done yet. I replaced the riser and water pressure on both engines are nearly the same and the temp rises to 165 on the bad engine and 158 on the good engine. But at the temp sensor on the manifolds, I am reading around 170 on the good engine and 190 on the bad engine. I'm taking it out today to and run it at speeds to see if it trips into guardian mode. We have gone through virtually everything in the cooling system. The only thing we don't know about is the flow through the exhaust manifold. Has anyone had an exhaust manifold plugged up, causing the engine to overheat? The only trouble shooting that we can think of is to do is check flow on the manifold on the good engine and try to compare that to the flow on the "bad" manifold.
 
Did you also replace the manifolds? I do them every 5 years
 
So you replace risers and manifolds every 5 years? That's not going to be cheap.
 
In salt water that’s about when things start to go and engines overheat. Much cheaper than the alternative.
 
I had an issue that may be similar to yours...
Read about it here...
http://clubsearay.com/index.php?thr...-port-and-stbd-emct-ovrht.80788/#post-1000100

What I believe the problem was that small chunks of rusted metal were circulating around in the in the manifold water and I'd think if a chunk hit the sensor that registers temp in the manifold it shorts the circuit and triggers the alarm. My manifolds were 10 yrs old at that time - 5 seasons in salt, 5 in brackish water. Never had any inkling of a temperature problem and after changing the manifolds and riser blocks the boat still runs at the same temp (according to smartcraft). Could be your problem too?
 
Have you done the water flow test? It would be good to know if you're at least getting the proper amount of cooling water TO the heat exchanger.
 
Yes, I have done the water flow test and it's good. New pump last year. Cleaned and reverse flowed the system of oil coolers and power steering coolers. Cleaned the heat exchanger. The only thing left in the system is the manifold and it didn't seem like it was flowing as well as it should but I don't know how a good one should flow. Off to get a new manifold.
 
Day tripper, I read your post and thanks. So changing the risers and manifolds did fix your problem but you think it was rust shorting the sensor? And your temps were the same before and after replacement? My bad engine is definitely 20 degrees hotter than the good engine. I replaced the risers but not the manifolds, going to try that next.
 
What was actual number of quarts you got in 15 seconds?

When you say "manifold and it didn't seem like it was flowing as well as it should"... earlier you said you didn't measure it. How are you measuring that? If you actually did?

Manifolds are typically easy enough to visually check before just replacing.
 
We pulled the manifold and when we took it apart we found the fitting right below the manifold that is a 1 way valve and a T was completely blocked with rust. The fitting is stainless but it was filled with rust that settled down to the lowest point. We are replacing both manifolds and risers on both engines. A very expensive repair.
manifold1.JPG
manifold2.JPG
 
Same crap I found in mine but to a lesser extent. That fitting on mine was not jammed up like yours.
 
I just bought one new manifold so I can get going but realized that I have Seacore system. Does anyone know if there are special "Seacore" manifolds? I didn't see anything listed on the Mercury parts site.
 
I believe seacore refers to the outdrives, not the engines.
 
Yes, I contacted Mercury and there are no "Seacore" manifolds or risors. Interesting, there should be. I ran the boat yesterday and temps were right on. Running nice and cool. Now I need to get the other engine done.
 
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